What happens if I miss the Self Assessment deadline?

Filing even one day late costs an automatic £100 penalty, whether or not you owe tax. After three months, daily penalties of £10 start, up to £900. Separate late-payment penalties of 5% apply at 30 days, six months and 12 months, plus interest on the unpaid tax.

Two separate systems

Filing late and paying late are penalised separately, so you can be hit by both on the same return.

Late filing: £100 immediately. After three months, £10 a day for up to 90 days, a maximum of £900. After six months, a further 5% of the tax due or £300, whichever is greater. After 12 months, another 5% or £300, whichever is greater.

Late payment: 5% of the unpaid tax at 30 days, another 5% at six months and another 5% at 12 months, plus interest from the due date.

Worked example

Reasonable excuse

HMRC can cancel a penalty where you had a reasonable excuse, such as serious illness, a bereavement, or a failure of HMRC's own systems. Forgetting, or not having the money, does not usually qualify. You normally have 30 days from the penalty notice to appeal.

What to do

1. File now. The penalties are time-based, so every extra day costs more.
2. Pay what you can, and ask HMRC for a time to pay arrangement if you cannot pay in full.
3. Appeal within 30 days if you have a genuine reasonable excuse, with evidence.
4. If you missed the July payment on account, treat it as part of the same problem.

Common mistakes

• Assuming no tax due means no penalty.
• Waiting for perfect records before filing.
• Not appealing within the time limit.

Sources: GOV.UK: Self Assessment penalties